I suspect it has to do with the specific variety you are growing. There are many different varieties of catmint and the color does vary although it stays in the lavender to purply blue range. For instance, Nepeta mussinii is described as lavender, Nepeta m. "Dropmore Hybrid" is described as lavender blue, Nepeta "Walker's Low" and "Six Hills Giant" are described as blue-violet, Nepeta siberica "Souvenir d'Andre Chaudron" is described as medium blue, Nepeta subsessillis is described as deep blue. To some extent the color will depend on the growing conditions -- and on the eye of the beholder (some of the above look pretty much the same in color to me, but might seem differentiated to you). You might try shopping when the plants are in bloom so you can see what you are getting. I hope this helps!